This invention relates to a cell search method in a code division multiple access (CDMA) system.
In the manner which is well known in the art, a code division multiple access (CDMA) system is one of multiple access system techniques in a cellular system (mobile communication system) such as a mobile telephone system and a portable telephone system used when a plurality of stations simultaneously carry out communications by using the same frequency band. On the other hand, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, and so on, are used as other multiple access system techniques other than the CDMA system. In comparison with the other multiple access techniques, the CDMA system is advantageous in that it is possible to achieve a high frequency utilization efficiency and is possible to accommodate more users.
The CDMA system carries out multiple access by a spread spectrum communication which transmits a spectrum of an information signal by spreading the spectrum of the information signal into a wide band sufficiently wider than an original information band width.
The cellular system (mobile communications system) generally comprises a mobile communications terminal (which is also called a “mobile station”) MS and a plurality of base stations BS each of which attends to a cell. The CDMA system is adopted as an access system for third-generation mobile communication systems. In the CDMA system, the mobile station MS carries out, as a handover in a case where the mobile station MS moves between a moving source cell and a moving destination cell, a soft handover which combines an electric wave signal from a base station BS in the moving source cell with another electric wave signal from another base station BS in the moving destination cell. In order to perform the soft handover, the mobile station MS must carry out a cell search which searches the base station BS in the moving destination cell. That is, the “cell search” or a “cell search processing” means that the mobile station MS detects a connected cell.
In addition, in the CDMA system, in order to increase a capacity by suppressing and preventing transmission and reception of interference signals, sectorization (a directivity control technique) for dividing each cell into a plurality of sectors is carried out.
Although each base station always transmits a radio signal via a radio channel called a perch channel, the radio signal on the perch channel is transmitted using a peculiar spreading code in each cell (base station). Accordingly, each base station informs, using the perch channel, the mobile station of a spreading code used in peripheral cells of its own station. The mobile station periodically measures line quality (signal-to-interference ratio (SIR)) of a radio channel (the perch channel of the peripheral cells) which uses the informed spreading code and informs a network of a measured result. Such an operation is called a peripheral cell search. As a result, the network can determine that to which cell the mobile station is moving.
In prior art, the above-mentioned cell search processing is a processing which is necessary to the mobile station MS on starting communication. That is, if the spreading code and a spreading timing are not established by the cell search, the mobile station MS cannot carry out a demodulation processing. As a result, rapidness of the cell search processing is one of techniques desired in communications in the CDMA system.
In addition, various preceding arts related to the present invention are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 9-271,071 or JP-A 9-271071 describes “MOBILE COMMUNICATION EQUIPMENT” to shorten the initial synchronization of a mobile station, time requiring the judgement of existence in a range and long code synchronous acquisition time in a cellular system using a CDMA (code division multiple access)/TDD (time division duplex) system. The mobile communication equipment described in JP-A 9-271071 comprises a receiving base-band processing part, a synchronizing circuit, a receiving level detection circuit. The receiving base-band processing part reproduces the data of a perch channel from a received signal by using symbol timing obtained from the synchronizing circuit. The synchronizing circuit detects a unique word from the data of the perch channel and executes slot synchronous acquisition. The receiving level detection circuit judges, as the nearest base station, a base station using a short code having the highest receiving level out of obtained receiving levels. The base station inserts information related to frame timing and information related to a long code sort used for an outgoing communication channel into the perch channel and transmits these information. The synchronizing circuit receives the transmitted information, acquires frame synchronization and acquires long code synchronization by using the long code sort.
In addition, Japanese Unexamined Patent Publication of Tokkai No. Hei 10-126,830 or JP-A 10-126830 describes “MOBILE COMMUNICATION TERMINAL” to detect the change of receiving quality of peripheral cells and to instruct the soft handover to a mobile communication terminal by adding a receiving level measurement means to the mobile communication terminal to measure the receiving level of the signal received in a receiving band before the adverse diffusion in addition to a peripheral cell search constitution. In JP-A 10-126830, a mobile communication station acquires the spread code information which is used by peripheral cells and stores it in a spread code storage means. A peripheral cell search control means sets a cell search execution cycle and a receiving level measurement execution cycle to first and second timers, respectively. When the second timer counts up its counting, the peripheral cell search control means instructs a receiving level measurement means to measure the receiving level and decides the value of RSSI change of a signal included in a receiving band based on the measurement result of the receiving level measurement means. If the RSSI change is large, the spread codes which are used by the peripheral cells stored in the spread code storage means are successively set to a sequential inverse spread means. Then a receiving quality measurement means measures the SIR received from every peripheral cell.
Furthermore, Japanese Unexamined Patent Publication of Tokkai No. Hei 10-164,021 or JP-A 10-164012 discloses “CELL SEARCH CIRCUIT FOR CDMA” to improve an S/N and to suppress the increase of search time and a circuit scale by synchronously adding the time integral results of signals accumulated in a ring buffer and controlling the shift operation of a diffusion signal generator having a phase shift function based on the result. That is, the cell search circuit for CDMA disclosed in JP-A 10-164012 comprises a spreading signal generation circuit with a phase shift function, a multiplier for multiplying the output of the spreading signal generation circuit by an input signal, and a ring buffer for accumulating correlation results for the number of synchronous adding times for synchronous-adding the plural signals. A buffer for one signal in the ring buffer and an adder constitute an accumulator and it integrates time in the same degree as unit signal continuing time before spreading. Synchronous addition is to add time integral results for respective signals accumulated in the ring buffer by the combination of polarities which are previously decided, to judge the synchronous state of the spreading codes based on a synchronous addition result and to a synchronous shift operation when synchronism is not established so as to acquire synchronism.
In addition, Japanese Granted Patent Publication of No. 2,861,985 or JP-B 2861985 discloses “HIGH SPEED CELL SEARCHING SYSTEM FOR CDMA” to enable cell search at a high speed by making data obtained by forming an orthogonal code through the use of a polarity of plural symbols spread by a spreading signal as a down signal and performing common-mode addition by means of a correlation device with the combination of polarities codes extending the plural symbols at the time of detecting. In JP-B 2861985, a correlating part is provided with a short code generating circuit and outputs a correlated value obtained by the correlating device to a ring buffer for common-mode addition by RAM. The buffer common-mode-adds the combination of the polarities the orthogonal code can make by an orthogonal code polarity control circuit inside a maximum value detecting circuit. The common-mode-added value of a group I and a group Q obtained as the result is transformed to power by a square sub circuit to be added to the maximum value detecting circuit and a chip counter and a symbol counter houses a chip position and a symbol position with maximum values and the polarity of the orthogonal code at the time in a maximum value register and outputs residual values to a long code identifier.
Now, attention will be directed to a case where the mobile station MS does not hardly move on starting communication from an end of previous communication and the mobile station MS lies in the same sector (or the same cell). In this event, the processed result of the cell search hardly should not change in the mobile station MS. However, as described above, the prior art performs the cell search processing in the mobile station MS on starting communication. Such an unnecessary cell search processing merely delays an acquisition of the cell search result and it results in increasing of consumed power.
On the other hand, any of the above-mentioned publications takes no thought of conditions regarding the case where the mobile station MS restarts communication with almost no moving, in the similar manner as the prior art.